This invention relates to continuing discoveries and improvements over our earlier invention of U.S. Pat. No. 7,618,546 issued Nov. 17, 2009. That patent disclosed improved method and means for ozone oxidation through the addition of an iron(II) catalyst. The present invention is based upon the discovery that allows selective oxidation of organic substrates with weak C—H bonds in the presence of ozone to less than fully oxidized products, for example, oxidation to aldehydes as opposed to acids. There is a continuing desire to selectively oxidize, for example, alcohols with ozone under mild conditions in an environmentally friendly way; for example, to aldehydes without over complete over-oxidation to carboxylic acids. The invention has as its primary objective the filling of this need. The invention uses the same iron(II) catalysts in ozone oxidation, as described in our previous U.S. Pat. No. 7,618,546, which is incorporated herein by reference.
Large quantities of ozone are typically produced commercially in a modern electrical ozone generator. The passage of a high voltage, alternating electric discharge through a gas stream containing oxygen results in the breakdown of the molecular oxygen, to atomic oxygen. Some of the atoms of oxygen thus liberated can reform into ozone, while others simply recombine to again form oxygen. In order to control the electrical discharge, and maintain a “corona” or silent discharge in the gas space and avoiding as much as possible, arcing, a dielectric space or discharge gap is formed, using a dielectric material such as glass or ceramic. A ground electrode, constructed usually in 316L stainless steel (a material which has demonstrated high resistance to ozone oxidant) serves as the other boundary to the discharge space. This can be accomplished in many ways, but the most frequently employed geometry is that of the cylindrical dielectric (or Siemens Type) ozone generator. The cylindrical dielectric is more space efficient than other shaped and consequently more economical.
Ozone produced commercially for oxidation reactions is always produced as a gas, from air at concentrations between 1.5 and 2.0 percent by weight in air, or from oxygen at concentrations greater than 6% and up to 12% by weight. As ozone is highly reactive, and has a short half life, it is very difficult to store and transport. Consequently, ozone is normally generated on site for immediate use.
As described previously, the catalyst of this invention comprises iron(II). Iron is an abundant and chemically benign element that exists in multiple oxidation states for catalysis. The source of iron(II) for use as an oxidant in this invention can be many of the commercially accessible inorganic salts including, but not limited to, tetrafluoroborate, hexafluorophosphate, perchlorate, trifluoro-methane sulfonate, sulfate, and combinations thereof. The chloride and bromide salts, however, are not useful. A preferred ferrous salt for this purpose is tetrafluoroborate. The iron salts are typically purchased in solid form, then combined with acetonitrile to form a dilute solution.
The ferrous salt is used in a concentration that should be substantially less than that of the ozone. While the concentration of ozone during oxidation is generally fixed due to solubility limits of ozone, in rough terms, the concentration of ozone is preferably about 20-50 times higher than that of the iron(II) to provide an instantaneous and complete or nearly complete oxidation of the substrate. If insufficient iron(II) is used, the oxidation will still occur (as it would even without the iron catalyst), but the reaction may not be as fast or complete. If too much iron(II) is included, undesirable reactions occur with Fe(IV), resulting in an iron(III) that cannot be converted back to the iron(II) catalyst
The iron catalysts of this invention, like our previous one, can be used in any applications and/or substrates for which ozone is used as an oxidant.